The present invention is concerned with the carbon dioxide and air reactivity reaction occurring during carbon electrode use. As noted in U.S. Pat. No. 4,130,475, coke formed from petroleum refinery streams is classified as No. 1 or premium coke, and No. 2 or regular coke. Premium coke differs from regular coke by its predominant metallic, crystalline appearance and a low linear coefficient of thermal expansion (CTE).
Both types of coke are used for the manufacture of carbon electrodes utilized in industry. The steel industry uses electrodes formed from premium coke in electric arc furnaces, while the aluminum industry uses electrodes formed from regular coke to produce aluminum. The description of such use in making aluminum can be found in Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 1, page 941.
Prior to the formation of electrodes, the refinery petroleum coke must first be calcined. Calcination usually occurs with temperatures in excess of 2200.degree. F., preferably above 2500.degree. F. The calcination densifies the coke and removes volatile matter while changing the carbon to hydrogen ratio.
The coke exiting the calciner at these high temperatures must then be cooled. This is typically accomplished in a cooler wherein water is sprayed onto the coke. The water by evaporation cools the coke to a suitable temperature. This calcined, cooled coke is then formed into electrodes.
As noted in the Kirk-Othmer reference, aluminum is formed by the electrolysis of alumina wherein the oxygen from the alumina combines with the carbon of the electrode to form carbon dioxide. A second and unwanted reaction occurs when the carbon dioxide again reacts with the carbon to form carbon monoxide. While 0.45 lbs. per lb. of aluminum is utilized for the electrolysis, 0.05 lbs. of carbon per lb. of aluminum is lost in forming carbon monoxide. The ability of a carbon electrode to react with CO.sub.2 to form carbon monoxide is known as the carboxy reactivity.
In the steel industry, electrodes are used in arc furnaces for melting of steel. A similar unwanted reaction occurs with air reacting with the carbon to form carbon oxides. Typically, 15-25% of the carbon is lost to this air reactivity reaction.
U.S. Pat. No. 3,320,150 notes that high reactivity can sometimes be due to the type of binder material or binder coke that is used and thus suggests a decrease in reactivity by utilizing a fine coke with a reactivity lower than the binder coke. U.S. Pat. No. 3,454,363 discloses the removal of metal contaminants from coke to lower reactivity. It is noted that the presence of cations is detrimental to some of the potential applications of coke, and thus the coke is contacted with a cation exchange resin to remove cations and thus lower reactivity. Ser. No. 107,988 filed Dec. 28, 1979, now abandoned, discloses a method of reducing coke reactivity by treatment of the quench water during the coking stage to remove such cations prior to any deposit on the coke.
It has been found that if the water used to quench calcined coke is treated with phosphoric acid, a lower reactivity and thus a better product can be obtained.